Operational amplifiers, instrumentation amplifiers, difference amplifiers, low drop-out regulators, voltage references and other similar types of devices typically are trimmed during or after manufacture to improve the precision and accuracy of the devices. Some of the main objectives for trimming techniques include the correction of offset, gain and temperature drift, among other parameters, in the devices. Modem trimming techniques for the initial production adjustment of offset, gain and other device parameters generally comprise some type of on-chip digital memory, for example in the form of EPROM or fuse links. During the trimming process, test circuitry is often used to measure the device parameters to make a determination as to how many and which of the specific memory cells should be programmed from the initial value, i.e., should be programmed from a “zero” or “low” state to the opposite “one” or “high” state.
Once the determination is made as to how many and which of the specific memory cells should be programmed, the information from the test circuitry must be transferred to an internal programming cell on the chip or device. To transfer this information, dedicated pins are used to facilitate communication. For serial communication protocols, at least two dedicated pins are necessary, while other devices and protocols can require more.
The requirement for additional dedicated pins can be a problem if the device under testing or trimming comprises a low pin count part with a standard pin output configuration, such as conventional operational amplifiers, voltage references, low drop-out regulators, and other like devices. Thus, controlling the trimming procedure in such devices having limited pin count configurations can be difficult.